The plasma membrane (outer membrane) of intact eukaryotic cells is characterized by a highly organized structure. This high level of membrane organization is determined, among others, by the molecular structure of the specific lipids constituting the membrane; the ratio between the various lipid species from which the membrane is composed; the distribution of the phospliolipids between the outer and inner leaflets of the membrane; and by the membrane protein constituents.
While maintenance of the high level of plasma membrane organization is fundamental to normal cell physiology, substantial perturbations and alterations of the normal organization of the cell plasma membrane (PNOM) occur in numerous physiological and pathological conditions, and are characterizing a plurality of diseases. Such alterations and perturbations may be evident both at the morphological level (membrane blebbing observed in cells undergoing apoptosis) and at the molecular level. PNOM includes, among others, scrambling and redistribution of the membrane phospholipids, with movement to the cell surface of aminophsopholipids, mainly phosphatidylserine (PS) and phosphatidylethanolamine (PE), which are normally restricted almost entirely to the inner leaflet of the membrane bilayer, and reciprocal movement of sphingomyelin (SM) and phosphatidylcholine (PC) from the outer leaflet to the inner leaflet of the membrane. This redistribution is referred herein as loss of cell membrane lipid asymmetry (CMLA). In addition to CMLA loss, PNOM is also often associated with reduction in the level of packing of membrane phospholipids and an increase in membrane fluidity.
These alterations play an important role in rendering the cell surface a catalytic platform for the assembly of several clotting factor complexes, such as the tenase and prothrombinase protein complexes. Accordingly, platelet activation is associated with the platelet membrane undergoing PNOM, and these alterations constitute an important factor in normal blood coagulation, as well as in the initiation and/or propagation of abnormal, excessive blood clotting in numerous disorders. These disorders include, among others, arterial or venous thrombosis or thrombo-embolism [e.g., cerebral stroke, myocardial infarction, deep vein thrombosis (DVT), disseminated intravascular coagulation (DIC), thrombotic thrombocytopenic purpura, etc.], unstable atherosclerotic plaques, sickle cell disease, beta-thalassemia, anti-phospholipid antibody syndrome [among others in systemic lupus erythematosus (SLE)], and disorders associated with shedding of membrane microparticles, e.g., neurological dysfunction in association with cardiopulmonary bypass.
Apoptosis is another major situation in which alterations/perturbations of cell membrane take place. Apoptosis is an intrinsic program of cell self-destruction or “suicide”, which is inherent in every eukaryotic cell. In response to a triggering stimulus, cells undergo a highly characteristic cascade of events of cell shrinkage, blebbing of cell membranes, chromatin condensation and fragmentation, culminating in cell conversion to clusters of membrane-bound particles (apoptotic bodies), which are thereafter engulfed by macrophages. PNOM is a universal phenomenon of apoptosis, it occurs early in the apoptotic cascade, probably at the point of cell commitment to the death process, and has also been shown to be an important factor in the recognition and removal of apoptotic cells by macrophages.
A strong correlation has been recently drawn between PNOM and the potent procoagulant activity of apoptotic cells. PNOM in apoptotic endothelial cells, such as those occurring in atherosclerotic plaques, probably plays an important role in the pathogenesis of thrombotic vascular disorders.
Since apoptosis or thrombosis each has an important role in the majority of medical disorders, it is desirable to have tools for detection of these biological processes and targeting of associated cells. Compounds for selective binding to PNOM-membranes, potentially also performing subsequent entry into and accumulation within these cells having such PNOM-membranes (PNOM-cells) may therefore serve as an important tool for detecting and targeting of imaging agents or drugs to cells undergoing damage or death process, especially by apoptosis, or to platelets undergoing activation.